It is common practice to use an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide as a catalyst in the oxidative coupling of 2,6-di-t-butylphenol.
Japanese Patent Laid-open Publication No. 5(1993)-97740 discloses that the above alkali metal hydroxide can be used not only in the form of a solution but also in anhydrous or nonsolvent form.
However, when the alkali metal catalyst such as an alkali metal hydroxide is dissolved in water to thereby obtain an aqueous solution (e.g., 48% aqueous KOH solution) and used as the catalyst as mentioned above, the alkali metal catalyst falls in the form of solution drops to the bottom of the reaction vessel because the solubility of alkali metal catalyst in 2,6-di-t-butylphenol is low. Thereafter, water vaporization occurs, so that the alkali metal catalyst cannot be homogeneously dispersed in the 2,6-di-t-butylphenol placed in the reaction vessel.
Therefore, problems have been encountered such that the oxidative coupling reaction rate is decreased and the amount of impurities is increased to thereby lower the yield of 3,3',5,5'-tetra-t-butylbiphenol and cause an aeration port of the reaction vessel to have an increased likelihood to clog up.
The occurrence of the above heterogeneous dispersion of alkali metal catalyst in the reaction system disenables the continuous oxidation reaction.
A large amount of alkali metal catalyst must be used for preventing the lowering of oxidative coupling reaction rate. However, at that time, it is also necessary to add a large amount of acid compound for neutralization.
With respect to the purified 2,6-di-t-butylphenol, the phenol content thereof is generally not greater than 0.1% by weight and, thus, it substantially does not contain phenol. Further, in the purified 2,6-di-t-butylphenol, the content of o-t-butylphenol is generally not greater than 0.1% by weight and the content of t-butylphenols other than o-t-butylphenol is generally not greater than 0.5% by weight.
In the oxidative coupling of the purified 2,6-di-t-butylphenol as a starting material of 3,3',5,5'-tetra-t-butylbiphenol in the presence of an alkali metal catalyst, an increase in the degree of oxidation of 2,6-di-t-butylphenol lowers the selectivity of desired 3,3',5,5'-tetra-t-butylbiphenol and increases the formation of by-products such as 3,3',5,5'-tetra-t-butyldiphenoquinone, 2,6-di-t-butylbenzoquinone and carbon dioxide contained in oxidation waste gas.
The above increase in the formation of by-products promotes the conversion of reaction products to a tar with the result that the color of the oxidation reaction mixture (3,3',5,5'-tetra-t-butylbiphenol containing by-products) deteriorates, and 3,3',5,5'-tetra-t-butylbiphenol is debutylated to thereby deteriorate the quality (purity and color) of obtained 4,4'-biphenol.
On the other hand, the crude 2,6-di-t-butylphenol obtained by reacting phenol with isobutylene in the presence of an aluminum phenoxide catalyst generally contains phenol, o-t-butylphenol, t-butylphenols other than o-t-butylphenol and 2,4,6-tri-t-butylphenol and 2,6-di-t-butylphenol in respective amounts of up to 1% by weight, up to 5% by weight, up to 3% by weight and 73 to 82% by weight of 2,6-di-t-butylphenol. When a large amount of starting material phenol and by-product o-t-butylphenol remain in the phenol butylating reaction mixture, the yield of 2,6-di-t-butylphenol decreases.
In the oxidative coupling of the crude 2,6-di-t-butylphenol as a starting material of 3,3',5,5'-tetra-t-butylbiphenol in the presence of an alkali metal catalyst, as in the oxidative coupling of purified 2,6-di-t-butylphenol, an increase in the degree of oxidation of 2,6-di-t-butylphenol lowers the selectivity of desired 3,3',5,5'-tetra-t-butylbiphenol and increases the formation of by-products such as 3,3',5,5'-tetra-t-butyldiphenoquinone, 2,6-di-t-butylbenzoquinone and carbon dioxide gas. Furthermore, the amount of products of oxidative coupling of 2,6-di-t-butylphenol with t-butylphenols mixed in the crude 2,6-di-t-butylphenol is increased.
Moreover, when 4,4'-biphenol is produced from 3,3',5,5'-tetra-t-butylbiphenol regardless if either the purified 2,6-di-t-butylphenol or the crude 2,6-di-t-butylphenol is used as the starting material of, trihydroyxbiphenyl occurs in the 3,3',5,5'-tetra-t-butylbiphenol debutylating reaction mixture. Additionally, an increase in the degree of oxidation of 2,6-di-t-butylphenol would increase the occurrence of trihydroxybiphenyl and invite deteriorations of the purity and color of 4,4'-biphenol.
Therefore, in the common oxidation, the oxidative coupling of 2,6-di-t-butylphenol must be conducted while restricting the degree of oxidation of 2,6-di-t-butylphenol at 75 to 85 mol % so that the formation of trihydroxybiphenyl in large amount can be prevented.
For obtaining the starting material of 3,3',5,5'-tetra-t-butylbiphenol which ensures production of high-quality 4,4'-biphenol while minimizing the formation of the above by-products, the degree of oxidation of 2,6-di-t-butylphenol must be held low while preventing the amount of 2,6-di-t-butylphenol in the oxidative reaction mixture from becoming smaller than a given level.
However, when the degree of oxidation of 2,6-di-t-butylphenol is held low, the problem is encountered that the 2,6-di-t-butylphenol charged in the reaction vessel cannot be satisfactorily utilized in the production of 3,3',5,5'-tetra-t-butylbiphenol from 2,6-di-t-butylphenol.
The inventors have made extensive and intensive studies with a view toward solving the above problems. As a result, it has been found that, in the catalytic oxidative coupling reaction of 2,6-di-t-butylphenol, the solubility of alkali metal catalyst in 2,6-di-t-butylphenol is markedly increased by the use of the alkali metal catalyst in combination with alkylphenols such as t-butylphenols or a liquid mixture of alkylphenols such as t-butylphenols and phenol, so that the oxidative reaction is homogeneously advanced to thereby enable producing 3,3',5,5'-tetra-t-butylbiphenol with high oxidation efficiency with the reduced occurrence of by-products such as carbon dioxide gas and 2,6-di-t-butylbenzoquinone. Moreover, it has been found that the oxidative coupling of purified 2,6-di-t-butylphenol or crude 2,6-di-t-butylphenol together with a given amount of phenol, o-t-butylphenol or other t-butylphenols in the presence of an alkali metal catalyst enables suppressing the formation of by-products such as 3,3',5,5'-tetra-t-butyldiphenoquinone, 2,6-di-t-butylbenzoquinone and carbon dioxide gas (impurities) even if the degree of oxidation of 2,6-di-t-butylphenol is increased to, for example, about 95%, and that the occurrence of trihydroxybiphenyl in the 3,3',5,5'-tetra-t-butylbiphenol debutylating reaction mixture can be inhibited in the production 4,4'-biphenol from the above 3,3',5,5'-tetra-t-butylbiphenol. The present invention has been completed on the basis of the above findings.